The present invention relates to analysis of core samples of reservoir rock in a laboratory, and more particularly to a method for high precision measurement of fluid saturation as a function of applied pressure in the reservoir rock samples. In a still more specific aspect this invention relates to a method employing noninvasive imaging techniques to directly determine fluid saturation distribution as a function of distance inside a porous rock core plug, and using this saturation for deriving a complete set of capillary pressure curves.
Knowledge of capillary pressure for each specific rock/oil/water combination present in reservoir rocks is highly important for predicting potential hydrocarbon recovery from the reservoir. Capillary pressure data, which is a measure of the interaction between fluids and the rock pore surface, includes determination of data that include development of complete positive and negative primary drainage, imbibition and secondary drainage curves. The strength of capillary interaction varies with the fluid saturations, the interfacial tension between the fluids, the pore structure, and the wettability of the pore surfaces. A measure of capillary pressure in a core sample can be calculated from the force exerted by the density difference between retained fluids at each height above or below a free water level in a centrifuged core. Knowing the fluid saturations as a function of the height above and below the free-water level permits the determination of a capillary pressure curve for any given reservoir rock/fluid system. Knowledge of the capillary pressure hysteresis, also called scanning curves, for a reservoir rock can be obtained in a laboratory. However, in practice this has been difficult to achieve, and is of uncertain accuracy or extremely time consuming to experimentally determine.
Capillary pressure curves are typically determined by either mercury intrusion, porous plate/membrane or centrifuge methods. Mercury intrusion, although rapid, provides questionable results due to the use of mercury in a vacuum to mimic water/oil behavior. Its methodology limits the technique to primary drainage and possible positive imbibition capillary pressures. Porous plate/membrane methods can generate all the capillary pressure curves, but to reach apparent equilibrium saturation can take days to months per pressure point for five to eight data points. This delays the availability of the results and limits the number of tests that can be made. The lengthy experimental time also increases the chance of a mechanical failure. The centrifuge method is normally used to determine drainage or negative imbibition curves. However, this method requires several centrifuge runs, which are time consuming such that it takes days to months to complete a test. A major limitation of the centrifuge methods is that it requires an assumed model to predict saturation distribution inside the core from the amount of liquid expelled. Accordingly, only an indirect or assumed or calculated measure of saturation at the inlet face of a rock plug is provided based on the amount of fluid expelled from the rock. Numerous methods over the past fifty years have been proposed for approximating the inlet saturation from centrifuge effluent volumes, but in every case, the model chosen influences the results, especially in the shape and location of the transition zone.
Accordingly, it is an object of this invention to derive complete positive and negative capillary pressure curves that closely mimic reservoir mechanism.
It is a more specific object of this invention to derive a capillary pressure curve that is determined from a measured oil/water saturation profile under a known pressure gradient as measured from a free-water level in a reservoir rock core sample.
A still more specific object is to establish an oil/water saturation distribution in a reservoir rock plug, via known fluid differential pressure by centrifuging.
Yet another object is to provide a method for deriving capillary pressure curves using noninvasive imaging techniques for determining fluid saturations, where the method is rapid, accurate and robust.
Still, another object of this invention is to provide methodology for capillary pressure measurement of rocks having a great disparity in petrophysical properties.
According to the present invention, the foregoing and other objects and advantages are attained by directly measuring an oil/water saturation profile within a porous rock core plug. The saturation profile, which includes a free water level, is established under a known pressure gradient induced by centrifuging, and is measured using image mapping techniques. The free water level is established at a desired position within the length of the core plug and provides a point of zero capillary pressure to facilitate measurement of both positive and negative capillary pressures.
In a preferred embodiment, oil and water saturation profiles within the rock plug are locked in place by freezing the oil phase while centrifuging, so that imaging the plug outside of the centrifuge allows direct mapping of the saturation profiles. In this manner both positive and negative portions of a capillary pressure curve are simultaneously obtained by calculating capillary pressure with reference to the free water level in the core plug as a point of zero capillary pressure. Complete capillary pressure curves are developed, which include a primary drainage curve of a core plug initially 100% saturated with water, a primary imbibition curve of a core plug initially at an irreducible water saturation, and a secondary drainage curve of a core plug initially at a residual oil saturation.
More specifically, the method for developing the primary drainage capillary pressure curve includes the following steps: fully saturating a generally cylindrically shaped core plug of porous rock with water, arranging the 100% water saturated core plug in a centrifuge holder and adding water and oil to the centrifuge holder so as to position a free water level encircling a cross section of the plug at a desired location along the length of the saturated core plug. Centrifuging the fully saturated core plug under oil and water for a predetermined length of time at a temperature above the melting point of the oil to establish the free water level within the plug, which then contains distributed water and oil. The core plug is cooled in the holder while still centrifuging to a temperature for solidifying the oil but not the water. Accordingly, distribution of fluids in the rock plug is fixed as the rock plug is removed from the centrifuge for imaging its saturation profile. Capillary pressure for each point of the saturation profile is then calculated from the force exerted on the retained fluid above and below the free water level for producing a primary capillary drainage curve for the core plug, which includes both positive and negative portions. Similar methods are used to obtain imbibition and secondary drainage capillary pressure curves, which each start with the core plug at an appropriate saturation state.
In another aspect, novel apparatus associated with the present invention includes a sample holder made of nylon for use in magnetic resonance imaging (MRI) or other noninvasive imaging devices. The holder, which is pressure sealed, is constructed to withstand high centrifugal forces encountered in a high speed centrifuge. The holder includes a sealable cup-like outer cylindrical member for holding an inner assembly. The inner assembly, which is used to centrally hold the core plug in the cup-like member, includes a circular bottom end piece, a circular top end piece, and three bar-like pieces which are longitudinally connected between the end pieces. The bar-like pieces have a width sufficient to provide a bulk annular volume between the core plug and outer cup-like structure of the holder that allows the core plug to be immersed in liquid while centrifuging.
The method and apparatus of this invention, is used to determine oil/water capillary pressure curves within a porous rock by establishing a freewater level in a core plug while centrifuging, and then freezing the oil phase while still centrifuging to facilitate saturation measurements on the core plug outside of the centrifuge. This method greatly reduces the time required to generate a set of capillary pressure curves including: equilibrium primary drainage, primary imbibition, and secondary drainage curves. Additionally, the method of this invention may reduce or eliminate many of the major limitations of other methods such as: determining the proper shape of capillary pressure curves; resolving questions on the impact of wettability/contact angle, saturation equilibrium, and boundary conditions. Further the method, which is applicable to diverse rock types, is accurate, rapid and robust.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein there is shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is suitable for other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.